SDL joystick support.
This is the lower-level joystick handling. If you want the simpler option, where what buttons does what is well-defined, you should use the gamepad API instead.
The term "instance_id" is the current instantiation of a joystick device in the system, if the joystick is removed and then re-inserted then it will get a new instance_id, instance_id's are monotonically increasing identifiers of a joystick plugged in.
The term "player_index" is the number assigned to a player on a specific controller. For XInput controllers this returns the XInput user index. Many joysticks will not be able to supply this information.
SDL_GUID is used as a stable 128-bit identifier for a joystick device that does not change over time. It identifies class of the device (a X360 wired controller for example). This identifier is platform dependent.
In order to use these functions, SDL_Init() must have been called with the SDL_INIT_JOYSTICK flag. This causes SDL to scan the system for joysticks, and load appropriate drivers.
If you would like to receive joystick updates while the application is in the background, you should set the following hint before calling SDL_Init(): SDL_HINT_JOYSTICK_ALLOW_BACKGROUND_EVENTS